1. Field of the Invention
The present invention relates to methods of synchronizing a receiver in a coded data transmission.
2. Prior Art
In data signal transmission, coding is often employed, for instance to enable reducing the required transmission energy per data bit for the same error rate. Reducing the transmission energy per data bit, however, leads to an impairment in the synchronization of the demodulator, which is already problematic anyway in particular in higher-stage modulation methods. In that case, one possibility for improvement is to use estimated values c".sub.i of the data decoder for the channel symbols c.sub.i actually transmitted. Such a method is described in reference 1, a paper by Ungerboeck, "Channel Coding with Multilevel/Phase Signals"), IEEE IT-28, No. 1, January 1982, pages 55-67, especially FIG. 17).
However, this method is problematic in two ways. First, these estimated values are present only in delayed fashion; that is, stability problems can arise in the close-loop control circuit, or control loop, comprising the demodulator and the data decoder that is created by the feedback. Second, the data decoder means a high-level nonlinearity with memory in the control loop, making the initial synchronization problematic, and under some circumstances the closed-loop control circuit can remain for relatively long periods in unstable states of equilibrium, known as "false locks".